Depression can be encountered concurrently with chronic medical illnesses. The low mood and its associated symptoms may comprise a reaction to a protracted malady, an independent primary affective disorder, a consequence of characterological vulnerabilities highlighted by the medical illness, a side-effect of drugs employed to treat other conditions, or perhaps a direct result of the illness process. As one of the most frequent chronic medical conditions, diabetes mellitus can be expected to be associated with depression in a considerable number of instances. Previous efforts to characterize psychological factors accompanying diabetes have been marred by methodological problems, especially inadequate measuring devices and unreliable psychiatric diagnostic criteria. While the literature is suggestive, global conclusions cannot be drawn. A systematic description of the relationship between depression and diabetes, detailing incidence and prevalence of depression in diabetics, the effect of depression on blood glucose control, the applicability of standard diagnostic criteria for identifying depression in diabetics, and the fit of theoretical models of depression to such a population, is not available. This investigation proposes to examine, employing clearly delineated diagnostic criteria, instruments, and research subjects, the incidence, prevalence, and course of depression in diabetic adults. In this manner, those depressive symptoms least often confounded by poorly controlled diabetes and its complications, and therefore more reliable and specific for depression, may be identified. Five etiological models for depression will be studied, with the aim of determining those that may best serve to guide therapeutic efforts. The impetus of the research will be to assess how improved identification and treatment of depression in diabetics can lead to enhanced metabolic control, and thereby contribute to rendering optimal medical management of diabetes.